Let's just say Obama puts a nationwide ban on assault weapons. What will all these people be doing with the new AR's they just bought. It's not like they can hide them. It's not like the government doesn't know they bought them. What do you do if the government gives you a grace period to turn them in and if you don't you're going to be committing a felony. There are so many ways this can go down but none of them really look good for gun owners.

Australia did a national buyback. That's one way. Another is to add an extra tax on them when sold/transferred. All sales go through licensed dealer with background check. That'll weed out the "Black Ops" wannabes.